Sediment Rigidity and Strain Distribution on the Hikurangi Subduction Zone: Preliminary Findings from the HOBITSS Experiment

Abstract:

From May 2014 to June 2015, the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) experiment deployed a dense network of 35 Ocean Bottom Seismometers (OBS) and Bottom Pressure Recorders (BPRs) to study near-source slow-slip processes at the Hikurangi margin off the North Island of New Zealand. The experiment was timed to coincide with a Slow Slip Event (SSE) predicted to occur in 2014, which was fortuitously recorded by the array in October of that year. The shallow sediment structure beneath HOBITSS stations can be probed using measurements of the transfer function between seafloor displacement and pressure under infragravity wave loading (the seafloor compliance). We measure compliance functions at periods from 50-250s, which are sensitive to sediment shear velocity structure at depths to ~4km beneath HOBITSS stations. We invert these data for sediment shear velocity models using a Monte Carlo search over sediment layer shear velocity and thickness. Recent work by Collins, McGuire, and Wei (AGU, 2013) employed seafloor compliance measurements to investigate how variations in sediment elastic properties influence interseismic strain accumulation and coseismic displacement in the up-dip region of the Cascadia subduction zone. Their investigation concluded that areas in the up-dip region with larger amounts of low shear-modulus sediments accumulated more interseismic strain, potentially resulting in larger coseismic displacements. Here we present results of a similar investigation of the Hikurangi margin. We compare sediment shear structure at sites on the seaward section of the downgoing plate and near the deformation front to the measured vertical displacements resulting from the October 2014 Slow Slip Event, to investigate whether areas of greater sediment volume and/or lower sediment rigidity exhibited greater aseismic displacements during the SSE.